1. Field of the Invention
The present invention relates to an apparatus and method for controlling the white balance, and more particularly to an apparatus and method for controlling the white balance of a cathode ray, robe (CRT) projector suitable for displaying two or more different types of input signals.
2. Description of the Related Art
Referring to FIG. 1, in a conventional white balance control apparatus, a correction waveform generator 51 generates a defocus signal for defocusing the electron beam for a blue fluorescent substance of a CRT. The defocus signal is amplified by a focus adjusting amplifier 52. The diameter of the electron beam for the blue fluorescent substance is controlled by a focus adjusting coil 53 according to the defocus signal so that the electron beam is defocused.
The luminance characteristic of the blue fluorescent substance is lower than the luminance characteristics of the green and red fluorescent substances corresponding to the anode current of each fluorescent substance used in the CRT projector. Further, the afterglow characteristic of the blue fluorescent substance is different from the afterglow characteristics of the green and red fluorescent substances, that is, the afterglow time of the blue fluorescent substance is shorter than the afterglow time of the other fluorescent substances. CRT projectors using fluorescent substances having these characteristics reduce the color temperature of white in the white balance adjustment. To improve the color temperature of white, the electron beam for the blue fluorescent substance is conventionally defocused.
In the case wherein the conventional white balance control apparatus defocusing the electron beam for the blue fluorescent substance is used in a multi-scan type CRT projector, when the CRT projector displays a video signal whose number of scanning lines is low, the white balance adjustment is effective for improving the color temperature of white. Here, the number of the scanning lines represents the number of the horizontal lines per vertical period.
However, when the CRT projector displays a signal generated from a personal computer (PC) and the like whose the number of scanning lines is high, the density of electrons reaching each fluorescent substance is reduced and, as compared with the case of the video signal whose the number of scanning lines is low, the anode current per unit area of each fluorescent substance is reduced. For the luminance characteristic corresponding to the anode current reduction, the characteristic of the blue fluorescent substance is different from the luminance characteristics of the red and green fluorescent substances. In other words, the luminance characteristics of the red and green fluorescent substances reduce in proportion to the anode current reduction, while the luminance characteristic of the blue fluorescent substance almost does not reduce.
Therefore, when a signal having a high number of scanning lines is displayed, the set color temperature becomes high compared with the case wherein a video signal having a low number of scanning lines is displayed.
Further, since the focus of the electron beam for the blue fluorescent substance is defocused, the diameter of the electron beam for the blue fluorescent substance becomes thick compared with the diameter of the electron beams for the red and green fluorescent substances.
Here, a conventional white balance control apparatus having a switch circuit which switches between an exact focus mode where the electron beam is exactly focussed and an over-focus mode where the electron beam for the blue fluorescent substance is defocused, is disclosed in Japanese Patent Application Laid-Open No. 1-170170 and Japanese Patent Application Laid-Open No. 64-27385. Further, another conventional apparatus which independently controls the amplitude value of red, green and blue color signals is described in Japanese Patent Application Laid-Open No. 4-129397.